This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
The scenario of interest is where a UE (User Equipment) is operating in a given LTE (Long Term Evolution) cell either in idle mode or with one or more ongoing packet data services (all non-speech) where the LTE network or the UE does not support the IP Multi-media Subsystem (IMS) control plane functionality for call establishment. The lack of IMS support for call establishment means that whenever a call (i.e. speech) needs to be established the UE must stop being served by the LTE network and start being served by a GSM or UMTS network for the purpose of performing the control plane signaling required for establishing the call. In other words, unless a transition from the LTE network to a GSM or UMTS network occurs when the need for call establishment is first detected, it will not be possible to set up the call.